This invention relates to phenolic resins, and more particularly to novolac resin compositions.
Phenolic resins have been of commercial significance for many years and have found acceptance in a very wide range of applications. Examples of these applications include wood products (plywood, particle board, etc), insulating foams, composites, carbon--carbon composites, electrical laminates, photoresists for microlithography, resins for non-metallic honeycombs, molding compounds, and in reactive processing (RIM, pultrusion). Novolacs, the acid-catalyzed condensation products of phenols with less than 1 mole of formaldehyde per mole of phenol, constitute a well-known class of phenolic resins. They are heat stable and require a curing agent to effect polymerization. The most widely used curing agent is hexamethylenetetramine. During the curing of novolac with hexamethylenetetramine, large amounts of volatiles, predominantly ammonia, are evolved. Gas evolution during cure presents safety concerns, and also makes the fabrication of void-free moldings difficult.
Functional derivatives of phenolic resins are known which will cure without the evolution of volatiles. For example, cyanate esters of novolac resins have been cured without additives and without the evolution of volatiles. See, for example, S. Das, D. C. Prevorsek, and B. T. DeBona, Modern Plastics, 1990, 72; G. W. Bogan, M. E. Lyssy, G. A. Monnerat, and E. P. Woo, SAMPE Journal, 1988, 24 (6), 19. The synthesis of these resins requires an additional chemical step since the novolac resin must be derivatized using cyanogen bromide. Also, since the phenolic hydroxyl group is involved in the derivatization, these modified resins can no longer be considered phenolic resins. Epoxy resins, comprising the glycidyl ethers of novolac resins, are also well known in the art. Though these resins cure without evolution of volatiles, an additive, or curing agent, such as an amine, is required to effect cure. In addition, these epoxy resins display similar features to the cyanate esters described above: their synthesis requires an additional chemical step, and the reaction of the phenolic hydroxyl group results in a resin which can no longer be classified as a phenolic resin.
The condensation products of bisphenol A and chloromethylbenzocyclobutene are known, as disclosed in U.S. Pat. Nos. 4,960,956 and 4,994,548. The benzocyclobutenes are connected to the phenolic rings via a methylene bridge. The benzocyclobutene-substituted compounds are restricted to diphenolic compounds. The cured products of the resins described in these patents are moderately soluble in 1,2-dichloroethane.
It would be desirable to provide new phenolic resins which, on curing, provide polymers that have high glass transition temperatures and are essentially insoluble in organic solvents.